Automotive Steering Systems and Wheel Alignment Principles

1. The Steering System

The steering system is the mechanism for guiding and orienting the wheels, controlled by the driver. It must meet European safety standards, which regulate the time and maximum force applied to the steering wheel. This safety system works in conjunction with the suspension, ensuring comfortable, precise, and safe vehicle handling.

Mechanical Components of the Steering System:

• Steering Wheel
• Steering Column/Shaft
• Steering Box/Reducers
• Universal Joints (Cruceta)
• Steering Linkage/Arms
• Steering Knuckles/Spindles

To assist motion, the force applied by the driver on the steering wheel is multiplied by the steering ratio, generating the necessary torque in the steering shaft to turn the wheels.

Steering Ratio (Transmission Relationship):

The steering ratio (Rt = angle of steering wheel rotation / angle of wheel turn) is the relationship between the angle of rotation of the steering wheel and the angle at which the road wheels turn. Increasing the steering wheel's diameter while applying the same force increases the mechanical advantage, but results in a lower steering gear ratio (more turns of the steering wheel for a given wheel angle).

1.3. Steering Box Reducers

Steering box reducers reduce and transform the rotational movement from the steering shaft.

Rack and Pinion Steering Box:

This is the most common system used in small cars. Typically, the steering column and steering wheel are positioned behind the front axle. The slack between the pinion and the rack can be adjusted using a semi-bushing, which is tightened by a calibrated spring. Tightening this cap increases the force against the rack, correcting any clearance.

Worm Gear Steering Box:

This system features a worm gear that rotates a sector gear, allowing the steering of two-wheel drive vehicles, particularly those with a front axle where space constraints prevent the use of a rack and pinion system, such as SUVs and vans. Turning the worm gear with the steering shaft moves a nut, and its displacement transmits motion to a control arm that, in turn, moves the steering arms.

Types of Worm Gear Steering Boxes:

• Worm and Nut
• Worm and Sliding Nut
• Recirculating Ball Worm
• Globular Worm and Roller
• Worm and Toothed Sector

1.4. Steering Linkage Components

These components transmit the linear movement from the steering box to the steering knuckles/spindles. Rack and pinion systems typically have fewer components in their linkage compared to worm gear systems.

Key Linkage Components:

• Tie Rods: Steel rods with a threaded hole at each end, connecting to ball joints or steering arms.
• Rack End (Inner Tie Rod): These bars transmit movement from the rack and pinion steering box to the ball joints. They connect to the rack bar with a ball head, allowing for rotational and angular displacements. The other end attaches to the steering knuckle.
• Adjusting Sleeves: Used to modify tie rod length for wheel alignment.
• Ball Joints: Allow for flexible movement and connection between components.
• Steering Arms: Connect the tie rods to the steering knuckles.

4. Wheel Geometry and Alignment Angles

Toe (Parallelism, Convergence, Divergence):

This refers to the parallelism of the wheels on each axle. Its main objectives are to ensure the wheels rotate parallel (zero toe) when the vehicle is in motion and to compensate for deformations in elastic joints (silent blocks).

Camber Angle:

The inclination of the wheel from the vertical, measured in degrees. It can be positive, zero, or negative.

Kingpin Inclination (Steering Axis Inclination - SAI):

The angle formed by the steering pivot axis relative to the vertical axis of the wheel. It reduces scrub radius without relying on camber and promotes steering wheel self-centering. Kingpin Inclination and Camber are related by the steering knuckle and ball joints, and their sum is called the 'Included Angle'.

Caster Angle:

The angle between the steering pivot axis and the vertical axis when viewed from the side. Positive caster means the steering axis is tilted forward at the top (like a bicycle fork), promoting straight-line stability. Negative caster means it's tilted backward.